A touch sensor is a device for detecting position coordinates pointed by a finger or a pen, or occurrence or nonoccurrence of the pointing motion. At present, the touch sensor is routinely used in combination with a display such as a liquid crystal display (LCD) or a plasma display panel (PDP).
It will be appreciated that a user friendly man-machine interface may be realized by entering an output of the touch sensor to a computer, controlling display contents on the display or controlling a peripheral using a computer. Nowadays, the touch sensor is used extensively in our everyday life, such as in game machines, mobile information terminals, ticket vending machines, automatic telling machines (ATM) or car navigation systems. Moreover, with higher performance of computers and with coming into widespread use of networking environments, variegated services are offered by electronic appliances. Hence, there are expanding needs for the display provided with touch sensors.
As touch sensor systems, there are currently known a capacitive system, a resistive system, an infrared (IR) system, a surface acoustic wave (SAW) system or an electro-magnetic resonance system. The capacitive system is subdivided into a projected capacitive type and a surface capacitive type.
The surface capacitive type touch sensor includes a transparent substrate, a uniform transparent electrically conductive film, formed on its surface, and a thin insulating film formed on an upper surface of the electrically conductive film. The transparent electrically conductive film is termed an electrically conductive position detection film. In driving the touch sensor, an ac voltage is applied to each of the four corners of the electrically conductive position detection film. When the finger has touched the electrically conductive position detection film, a minor current will flow through the finger via a capacitor formed between the electrically conductive position detection film and the finger. This current flows from each of the respective corners to the touched point. A signal processing circuit detects whether or not touch has been made based on the sum of the currents, while calculating the coordinates of the touched position based on the ratio of the currents. Patent Document 1 discloses a basic apparatus in connection with the technique of the surface capacitive type touch sensor. Patent Document 2 shows pertinent known examples in connection with this technique.
The conventional practice has been to use the surface capacitive type touch sensor, having a transparent substrate as one of constituent elements, as the sensor is superposed on a display. In such case, there is raised a problem that the display is increased in thickness or weight, due to the thickness of the touch sensor itself, or the quality of demonstration is lowered due to the presence of a component overlying the display surface. Patent Document 3, for example, discloses a technique that addresses this problem. Specifically, Patent Document 3 discloses a liquid crystal display in which a surface capacitive position detection electrically conductive film is unified to a front or back surface of a color filter substrate. It has been felt to be desirable to provide the position detection electrically conductive film on a side more proximate to a polarization plate than to an ITO (Indium Tin Oxide) film, as a common electrode, in consideration of electrical effects, that is, noise, as will be described subsequently. By this formulation, a transparent substrate, so far needed apart from the liquid crystal display, may be dispensed with to enable reduction in weight and thickness as well as to prevent picture quality deterioration.
In Patent Document 3, the following description is made in connection with the noise shielding effect. The surface capacitive type touch sensor is inherently susceptible to noise. A liquid crystal display is susceptible to noise because of variations in the potential of the pixel electrode. However, with the liquid crystal display, the potential of the common electrode is fixed or inverted at a stated interval for driving the electrode. It is thus possible to use the common electrode (ITO film) disposed between the electrically conductive position detection film and a TFT array as a noise shield. That is, Patent Document 3 states that the common electrode that is provided between the electrically conductive position detection film and the pixel electrode and that is connected to a fixed potential, performs the role of a noise shield.
Patent Document 4 points out that, in a structure where the electrically conductive position detection film is unified to a color filter substrate, the capacitive coupling between the electrically conductive position detection film and the common electrode is much stronger than capacitive coupling between the electrically conductive position detection film and the human (effective capacitance), which should pose a problem. Patent Document 4 proposes a structure and a driving method that address this problem. In a structure proposed, an electrically conductive position detection film is unified on a first substrate to a protective plane layer, which protective plane layer is arranged between the electrically conductive position detection film and the common electrode. That is, there is disclosed a structure in which the protective plane layer is newly provided between the electrically conductive position detection film and the common electrode. In a driving method, a signal obtained on amplitude scaling or phase shifting a signal of the electrically conductive position detection film is delivered to the protective plane layer. This should lower the capacitive coupling between the electrically conductive position detection film and the common electrode.
Patent Document 5 discloses a setup including a liquid crystal display circuit, a position detection circuit, and a switching circuit. The liquid crystal display circuit delivers the voltage or the current for demonstration to a transparent counter electrode, and the position detection circuit detects currents flowing from a plurality of positions on the transparent counter electrode. The switching circuit provides for electrical connection of the liquid crystal display circuit or the position detection circuit to the transparent common electrode. Patent Document 5 states that the problem of deterioration of display quality may be overcome by temporally isolating a case where the transparent common electrode is used as a common electrode for display and a case where it is used as an electrically conductive position detection film and by alternately switching one of the two cases to the other and vice versa.
Patent Document 6 teaches a driving device for a display including a touch panel provided with an electrically conductive film. The driving device for the display includes a counter electrode driving means. During the non-displaying time such as during the vertical blanking period, the counter electrode driving means applies the same signal as that applied to the transparent common electrode of the touch panel to the counter electrode. Since the potential at the counter electrode is the same as that at the transparent electrically conductive film on the touch panel, the induced voltage at the touch panel, ascribable to the potential difference between the counter electrode and the transparent electrically conductive film, may be reduced to a level that does not affect position detection accuracy.